This application claims the benefit of Japanese Patent applications No. 2001-124314 and No. 2002-051175 which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a compact zoom lens system and, in particular, to a zoom lens system designed specially to be compact with its whole optical system, being suitable for such as a digital still camera
2. Related Background Art
In an image gathering system using a solid-state imaging device, in order to arrange a low-pass filter or a color correction filter, a lens system having a relatively long back focal length is required. Moreover, a lens system having a good telecentricity on an image side is required. In these days, compactness and low cost are also required to a lens system in addition to satisfying these requirements.
A three-lens-group zoom lens system having, in order from an object side, a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group having positive refractive power wherein zooming from a wide-angle end state to a telephoto end state is carried out by moving the first lens group and the second lens group has been proposed in Japanese Patent Application Laid-Open No. 10-293253. However, the zoom lens system proposed in Japanese Patent Application Laid-Open No. 10-293253 has drawbacks such as relatively large number of lens elements composing each lens group, relatively large total lens length, and higher manufacturing costs.
Moreover, Japanese Patent Application Laid-Open No. 2001-13408 discloses an optical system in which the number of lens elements composing a first lens group is reduced. However, a positive lens element is arranged on the most object side of a first lens group having negative refractive power, so that it has a drawback that the diameter of the lens system inevitably becomes large when the system is made to have a wider angle of view.
Furthermore, since the first lens group separates largely from the aperture stop in the wide-angle end state, the height of an off-axis ray incident to the first lens group becomes large, so that the diameter of the lenses composing the first lens group becomes large. As a result, the lens system has a drawback that the whole lens system becomes large.
The present invention is made in view of the aforementioned problems and has an object to provide a zoom lens system suitable for a image gathering system using a solid-state imaging device, having a zoom ratio of about three, a small total lens length, and superb optical performance.
According to one aspect of the present invention, a zoom lens system includes, in order from an object, a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group having positive refractive power. Zooming is carried out by varying distances between adjacent lens groups. The first lens group includes two negative lens elements and a positive lens element. The second lens group includes three lens elements consisting of a first positive lens element, a second positive lens element, and a negative lens element. The third lens group includes at least one positive lens element. The following conditional expression (1) is satisfied:
2.5 less than TL/(ftxc3x97fW){fraction (1/12)} less than 4.5xe2x80x83xe2x80x83(1)
where TL denotes the distance between the most object side lens surface of the zoom lens system and the image plane, fw denotes the focal length of the zoom lens system in a wide-angle end state, and ft denotes the focal length of the zoom lens system in a telephoto end state.